The present invention relates, in general, to phased array antennas and, in particular, to phased array antennas that require grating lobe suppression.
A phased array antenna is a plurality of sub-array antennas coupled to a common source or load in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.
A limited scan antenna system scans a narrow beam only a few beam widths. Grating lobe suppression is a difficult design task for limited scan antennas where sub-arrays are employed. Few techniques have been developed to reduce the level of spurious grating lobes. One approach is to use non-constant sub-array separations which disrupt the coherent summation of radiation in the grating lobe directions. However, the resulting side lobes are higher.
Another approach is overlapped sub-arrays that interleave the radiation elements. For a fixed sub-array separation, overlapping sub-arrays allow a larger sub-array aperture, resulting in a narrower beam width of the sub-array pattern. The grating lobes of the array can be placed completely within the side lobe region of the sub-array pattern, giving grating lobe suppression. This method works well when the radiation elements are relatively short in the vertical direction according to the orientation shown in FIG. 1
However, for long element arrays, the coupling between elements and, hence, the sub-arrays, due to interleaving, become stronger. The consequence is that the sub-array patterns are degraded resulting in lower gain and higher side lobes, and sub-array port-to-port isolation deteriorates.
It is desirable to provide a novel solution for a partially overlapped sub-array antenna approach, for both short and long element arrays, which provides high isolation between the sub-array ports and desired sub-array patterns can be achieved in a simple and low cost structure.